Physical Layer – Part 2 Data Encoding Techniques Computer Networks: Data Encoding

Analog and Digital Transmissions Figure 2-23.The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modems and codecs. Tanenbaum slide Computer Networks: Data Encoding

Data Encoding Techniques Digital Data, Analog Signals [modem] Digital Data, Digital Signals [wired LAN] Analog Data, Digital Signals [codec] Frequency Division Multiplexing (FDM) Wave Division Multiplexing (WDM) [fiber] Time Division Multiplexing (TDM) Pulse Code Modulation (PCM) [T1] Delta Modulation Computer Networks: Data Encoding

Digital Data, Analog Signals [Example – modem] Basis for analog signaling is a continuous, constant-frequency signal known as the carrier frequency. Digital data is encoded by modulating one of the three characteristics of the carrier: amplitude, frequency, or phase or some combination of these. Computer Networks: Data Encoding

Computer Networks: Data Encoding A binary signal Amplitude modulation Frequency modulation Phase modulation Figure 2-24. Tanenbaum slide Computer Networks: Data Encoding

Computer Networks: Data Encoding Modems All advanced modems use a combination of modulation techniques to transmit multiple bits per baud. Multiple amplitude and multiple phase shifts are combined to transmit several bits per symbol. QPSK (Quadrature Phase Shift Keying) uses multiple phase shifts per symbol. Modems actually use Quadrature Amplitude Modulation (QAM). These concepts are explained using constellation points where a point determines a specific amplitude and phase. Computer Networks: Data Encoding

Constellation Diagrams (a) QPSK. (b) QAM-16. (c) QAM-64. Figure 2-25. Tanenbaum slide Computer Networks: Data Encoding

Digital Data, Digital Signals [the technique used in a number of LANs] Digital signal – is a sequence of discrete, discontinuous voltage pulses. Bit duration :: the time it takes for the transmitter to emit the bit. Issues Bit timing Recovery from signal Noise immunity Computer Networks: Data Encoding

NRZ ( Non-Return-to-Zero) Codes Uses two different voltage levels (one positive and one negative) as the signal elements for the two binary digits. NRZ-L ( Non-Return-to-Zero-Level) The voltage is constant during the bit interval.   NRZ-L is used for short distances between terminal and modem or terminal and computer.  negative voltage 0  positive voltage Computer Networks: Data Encoding

Computer Networks: Data Encoding “Plain” NRZ Figure 2.6 P&D slide Computer Networks: Data Encoding

NRZ ( Non-Return-to-Zero) Codes NRZ-I ( Non-Return-to-Zero-Invert on ones) The voltage is constant during the bit interval.   NRZI is a differential encoding (i.e., the signal is decoded by comparing the polarity of adjacent signal elements.) 1  existence of a signal transition at the beginning of the bit time (either a low-to-high or a high-to-low transition)  0  no signal transition at the beginning of the bit time Computer Networks: Data Encoding

Computer Networks: Data Encoding Bi –Phase Codes Bi- phase codes – require at least one transition per bit time and may have as many as two transitions.  the maximum modulation rate is twice that of NRZ  greater transmission bandwidth is required. Advantages: Synchronization – with a predictable transition per bit time the receiver can “synch” on the transition [self-clocking]. No d.c. component Error detection – the absence of an expected transition can be used to detect errors. Computer Networks: Data Encoding

Computer Networks: Data Encoding Manchester Encoding There is always a mid-bit transition {which is used as a clocking mechanism}. The direction of the mid-bit transition represents the digital data. Consequently, there may be a second transition at the beginning of the bit interval. Used in 802.3 baseband coaxial cable and CSMA/CD twisted pair. 1  low-to-high transition 0  high-to-low transition Textbooks disagree on this definition!! Computer Networks: Data Encoding

Differential Manchester Encoding mid-bit transition is ONLY for clocking. Differential Manchester is both differential and bi-phase. Note – the coding is the opposite convention from NRZI. Used in 802.5 (token ring) with twisted pair. * Modulation rate for Manchester and Differential Manchester is twice the data rate  inefficient encoding for long-distance applications. 1  absence of transition at the beginning of the bit interval 0  presence of transition at the beginning of the bit interval Computer Networks: Data Encoding

Computer Networks: Data Encoding Bi-Polar Encoding 1  alternating +1/2 , -1/2 voltage 0  0 voltage Has the same issues as NRZI for a long string of 0’s. A systemic problem with polar is the polarity can be backwards. Computer Networks: Data Encoding

1 1 1 1 1 Unipolar NRZ Polar NRZ NRZ-Inverted (Differential Encoding) 1 1 1 1 Unipolar NRZ Polar NRZ NRZ-Inverted (Differential Encoding) Bipolar Encoding Manchester Encoding Differential Manchester Encoding Figure 3.25 Copyright ©2000 The McGraw Hill Companies Leon-Garcia & Widjaja: Communication Networks

Figure 2.7 Note –Manchester is wrong!! NRZI is also wrong!! P&D slide Computer Networks: Data Encoding

Analog Data, Digital Signals [Example – PCM (Pulse Code Modulation)] The most common technique for using digital signals to encode analog data is PCM. Example: To transfer analog voice signals off a local loop to digital end office within the phone system, one uses a codec. Because voice data limited to frequencies below 4000 HZ, a codec makes 8000 samples/sec. (i.e., 125 microsec/sample). Computer Networks: Data Encoding

Computer Networks: Data Encoding Multiplexing Time-Division Multiplexing (TDM) Frequency-Division Multiplexing (FDM) P&D slide Computer Networks: Data Encoding

Multiplexing (a) (b) A A A Trunk group A B B B MUX MUX B C C C C Copyright ©2000 The McGraw Hill Companies Leon-Garcia & Widjaja: Communication Networks Figure 4.1 Computer Networks: Data Encoding

Frequency Division Multiplexing (a) Individual signals occupy H Hz C f B A H (b) Combined signal fits into channel bandwidth A C B f Copyright ©2000 The McGraw Hill Companies Leon-Garcia & Widjaja: Communication Networks Figure 4.2 Computer Networks: Data Encoding

Frequency Division Multiplexing Figure 2-31. (a) The original bandwidths. (b) The bandwidths raised in frequency. (c) The multiplexed channel. Tanenbaum slide Computer Networks: Data Encoding

Wavelength Division Multiplexing Figure 2-32. Tanenbaum slide Computer Networks: Data Encoding

Time Division Multiplexing (a) Each signal transmits 1 unit every 3T seconds t A1 A2 B1 B2 C1 C2 3T 0T 6T (b) Combined signal transmits 1 unit every T seconds t B1 C1 A2 C2 B2 A1 0T 1T 2T 3T 4T 5T 6T Copyright ©2000 The McGraw Hill Companies Leon-Garcia & Widjaja: Communication Networks Figure 4.3 Computer Networks: Data Encoding

Computer Networks: Data Encoding Time Division Multiplexing Stallings slide Computer Networks: Data Encoding

Statistical Multiplexing On-demand time-division Schedule link on a per-packet basis Packets from different sources interleaved on link Buffer packets that are contending for the link Buffer (queue) overflow is called congestion ■ ■ ■ P&D slide Computer Networks: Data Encoding

Computer Networks: Data Encoding Statistical Multiplexing - Concentrator Stallings slide Computer Networks: Data Encoding

Pulse Code Modulation (PCM) Analog signal is sampled. Converted to discrete-time continuous-amplitude signal (Pulse Amplitude Modulation) Pulses are quantized and assigned a digital value. A 7-bit sample allows 128 quantizing levels. Computer Networks: Data Encoding

Pulse Code Modulation (PCM) PCM uses non-linear encoding, i.e., amplitude spacing of levels is non-linear. There is a greater number of quantizing steps for low amplitude. This reduces overall signal distortion. This introduces quantizing error (or noise). PCM pulses are then encoded into a digital bit stream. 8000 samples/sec x 7 bits/sample = 56 Kbps for a single voice channel. Computer Networks: Data Encoding

Computer Networks: Data Encoding Stallings slide Computer Networks: Data Encoding

Computer Networks: Data Encoding PCM with Nonliner Quantization Levels Stallings slide Computer Networks: Data Encoding

T1 System 1 1 2 MUX MUX 2 . . . . . . 22 23 24 b 1 2 . . . 24 b 24 frame 24 Copyright ©2000 The McGraw Hill Companies Leon-Garcia & Widjaja: Communication Networks Figure 4.4 Computer Networks: Data Encoding

Figure 2-33.T1 Carrier (1.544Mbps) T1 – a TDM System The T1 carrier (1.544 Mbps). Figure 2-33.T1 Carrier (1.544Mbps) Tanenbaum slide Computer Networks: Data Encoding

Computer Networks: Data Encoding Delta Modulation (DM) The basic idea in delta modulation is to approximate the derivative of analog signal rather than its amplitude. The analog data is approximated by a staircase function that moves up or down by one quantization level at each sampling time.  output of DM is a single bit. PCM preferred because of better SNR characteristics. Computer Networks: Data Encoding

Delta Modulation DCC 6th Ed. W.Stallings Computer Networks: Data Encoding